Haskell has printf without using lists-as-parameters. It accomplishes this by doing some very clever type-level trickery. The key idea is in the recursive definition of this instance of PrintfType:
instance (PrintfArg a, PrintfType r) => PrintfType (a -> r) whereYour snippet got cut off, care to paste the rest? I'm guessing it works by popping the format string and the first operand off the args, and then finding the first formatting escape in the format string and formatting it, then recurse.
Yeah, sorry. Here's the full snippet:
instance (PrintfArg a, PrintfType r) => PrintfType (a -> r) where
spr fmts args = \ a -> spr fmts (toUPrintf a : args)
spr :: String -> [UPrintf] -> t
Re:What is the advantage of currying?
by guard-of-terra on Saturday February 02, 2013 @09:07AM
(#5155977)
"the compiler still has to know how many parameters a function takes before it can be called"
I guess it's better viewed as: type system has to figure out whether a consecutive currying/calling will return a scalar/function of zero arguments.
OCaml famously (or notoriously) hacks this knowledge into the compiler, so the compiler knows about format strings and parses them. This is nicely typesafe, but means you cannot use varargs in your own functions, and there are odd restrictions on the format string. (You can use the camlp4 preprocessor, or lists-as-parameters, to get around this).